Secret signaling system



April E. Y. WEBB, 1R 2515,56?

SECRET SIGNALING SYSTEM Filed March 15,. 1941 3 Sheets-Sheet l zoom April 5, 1943- E. Y. WEBB, JR 2,315,567

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SECRET SIGNALING SYSTEM Filed March 15, 1941 3 Sheets-Sheet 3 All II 3 SMW/QW .wires of a hand telephone set in common It will be apparent from inspection of Fig. It at Patented Apr. 13, 1943 aerate? SECRET SIGNALDN'G SYSTEM Edwin Yates Webb, Jn, Atlanta, Ga., assignor of one-fourth to Herbert L. Davis, Washington,

Application March 15, .1941, Serial No. 383,611

l'lClaims.

The invention forming the subject matter of this application is an improvement on the portable secret telephone and radio system disclosed in-my U. S. Patent No. 2,206,590, July 2, 1940.

In its broadest form, the system is shown in the patent as reduced to its utmost simplicity for the purpose of keeping the apparatus as small and as light as possible. Where size and weight are not of prime importance, certain additions can be made to the patented circuit which advance the breaking of its secrecy into the realm of the impossible.

The main object of the invention is to provide a secret signaling system which includes transmitter-receiver units, adapted to be readily attached to, or detached from, any voice communi-. cation channe1 without requiring any special treatment of the channel or interference with its normal operation.

Another object of theinvention is to provide a system of secret signaling including any number of similar transmitter-receiver units, each capable of exchanging secret signals with each other under the control of only one of such units 'which is provided with additional mechanism of the system in concrete form.

- Inthe drawings, 7

Fig. 1 is a schematic diagram of one of the transmitter-receiver privacy units embodying the present invention: V s

Fig. 2 is an explanatory diagram illustrating the various stages in frequency changing involvedin the operation of this secrecy system;

Fig. 3 is a detail, to an enlarged scale, of one of the elements shown-in Fig. 1;

Fig. 4 is a fragmentary detail of a modified form of carrier generator; and

Fig. 5 is a schematic diagram of a phase-shift network forming one of the elements of said modified form of carrier generator.

In Fig. lthere is illustrated one of the aforesaid privacy units connected by a six-pole doublethrow switch S to the, transmission-reception when the switch S is 'in theiffofl' position, the

ordinary inter-phone ccnve'rfsation may be effected through the transmissioi iine I, reception I F-l eliminates all frequencies line 2 and the common line 3. When this switch is in the "on position, ordinary speech current enters the transformer T of the transmitting channel from the telephone transmitter (not shown) through line l,,switch arm 3, lead 5, pri- 'mary of transformer T, and lead 6 connected to the common line 3. i

In the output or transmitting channel, the band-pass filter F eliminates all frequencies below 400 cycles and above 2400 cycles; and, this selected'band, between 400 and 2400 cycles, is' impressed across the bilatera1 oxide modulator M-l, to which is also applied a carrier shifting continuously and irregularly between 2800 and 3100 cycles. In this modulator, the 400 to 2400 cycle speech input is mixed with the shifting carrier to cause voice inversion, low frequencies becoming high and high frequencies becoming low, while at the same time they are shifted 300 cycles to and fro by the shifting 2800 to 3100 cycle modulating carrier.

The original speech input is prevented from passing into the output circuit by adjusting the balance of the two branches ofthe modulator M--l through the potentiometer P. The shifting carrier is balanced out of the filter and output circuit by means of the potentiometer P-Z which permits a high degree of balance so that the electrical center of the filter and output circuit can be obtained to secure exactly equal potentlals across the filter terminals.

below 400'cycles and above 2700 cycles. I

The shifting inverted voice band of 400 to 2700 cycles passes through the transformer T-I, am-

, plifier V, transformer T-Z and lines I and 8 to the output ends of the transmission line I and privacy unit is disconnected- 1mm the line and V-2, the frequency of which is caused to vary between 350 and 387 cycles by rotation of the, rotatable elements of the varlablecondenser (!-lv driven by the motor M. The variations in the frequencies of this oscillator may be made completely random and irregular by shaping the condenser plates in irregular 'patternsand by irregularly varying the speeds of the motor by any suitable means.

The 350. to 387 cycle low frequency passes The filter through the transformer T-S into the harmonic generator G, where the fundamental frequency is balanced out, and even multiples thereof are created by the distorting action of the full-wave copper oxide rectifier. From the output of this harmonic generator G, the filter F2 selects the proper harmonic of the fundamental frequency which lies in the band between 2800 and 3100 cycles. passes through the transformer T-t, amplifier Vt, and transformer T5 into the control rheostats R--i and R-2, and thence to the input of the modulator lVf-i and to the input of the demodulator M--2 inthe receiving section of the unit.

It is evident that in order to re-invert this shifting band of frequencies, the demodulating frequency must shift in exact synchronism, for otherwise the demodulated frequencies would be unintelligible. Obviously, any eavesdropper with an independent demodulating oscillator will hear speech frequencies constantly varying in pitch, in a rapid and random manner, which will be impossible to understand. Any variation in this independent oscillator would only add to the confusion, since it would add a second varying demodulating frequency. Therefore, the only way in which this varying inverted voice band can be demodulated is by using the original shifting frequency in the demodulator.

As shown clearlyin Fig. 1, the receiving channel of the section containing the demodulator M--2 is substantially the same as the transmitting channel of the transmission section containing the modulator l\/Ii, with the various This shifting band of frequencies their having their switches moved to receiving positions R, with their tubes V2 operating as amplifiers for the low frequency current received through the low pass filters F-3 from the line.

At each distant unit, the inverted shifting voice band (400-2700 cycles) passes through the receive line 2, switch arm i8, line H, primary of transformer Tll, and common lines 8 and 3. From the transformer T-l of the receive section, this inverted shifting voice band passes through the band-pass filter F-d to the demodulator M-2. At the same time, the shifting low frequency current (350387 cycles) is conducted by the receive line i? and common line through the low pass filter F-3 and transformer Ti5 to the circuit of the amplifier V 2. Therefore, instead of generating 350-387 cycle current, the

- receiving unit amplifies that low frequency curelements arranged in the opposite direction of transmission to re-invert the inverted voice band.

received from a distant unit. At the same time that the 350-387 cycle current is fed. to the harmonic generator G, it is also fed through the transformer T-B to the low-pass filter ,F-3 which is connected across the lines H and I2; the line ll being connected by a switch I 3 and line It to the transmission line I, and the line I? being connected directly to the common line 8.

.The result is that this low frequency current (350-887 cycles) is transmitted also to the distant terminals along with the shifting inverted voice band of 400 to 2700 cycles.

Three switches I3, i5 and I6 are used in the control or master unit shown in Fig. 1, and are ganged to move from the transmitting positions T, shown in full lines, to the receiver positions R, shown by'dotted lines, the switch l3 being the motor control switch. The switch I5 is used to I convert the oscillator V-2' to an amplifier in all distant units intendedfor control by the master (oscillating) unit shown in Fig. 1

In the oscillating position (Fig. 3) the switch- ISby-passes the larger resistanceR-3 and inserts the smaller resistance Rl in the plate circuit. When the switches are brought to receive positions-R, the condenser-driving motor M -is stopped; the line H is connected through the line I! and switch arm Is to the'output end of the receive line 2; and the larger resistance R-3 replaces the smaller resistance R-4 in the plategrid feed back circuit of the tube V--2. The latter operation causes the oscillations to cease, but increases the amplification of the tube by increasing its effective plate potential, at the same time affording enough. feed back to increase still further the amplification of the tube without producing oscillation. Hence, the units remote fromthe oscillating unit are to be considered as rent generated by the distant oscillating unit. After this received low frequency current is amplified, it passes through the transformer T-3, harmonic generator G, bandpass filter F-2, transformer TB, amplifier V3, and transformer T5 into the control rheos'tats R.i and R-2; and, thence, to the input of the modulator M-i and demodulator M2 where it now meets the incoming inverted shifting voice band.

Since the shifting voice band and the shifting modulating carrier are in exact synchronism,

being-generated by the'same source (the oscillaband thus passes out of the demodulator cir-' cuit in its original form and without any evidence of variation in pitch.

From inspection of Fig. 1, it will be apparent that when this received 350-387 cycle band is passed into the harmonic generator G and demodulator M-2, it is also passed into the modulator circuit where it-is mixed with the voice and transmitted to the line in the form of a shifting voice band'of 400-2700 cycles. Therefore, any of these privacy units remote from the control (oscillating) unit may be used without change to transmit speech to and receive speech ,from the terminal acting as master unit, as well as any other units connected to the communicaticns circuit.

The fact that one unit generates the low frequency current for one or. more distant units does not imply that any push-to-talk" principle is involved.

The system is asimultaneous twoway means of communication. When the control (oscillating) unit is' used for reception, the three switches l3, I5 and I8 remain in their transmitting positions T. In this case, however, the incoming signal energy flows through the receive line 2, switch arm l8, line H, primary of transformer T-l, common line 33, secondary of transformer T2, transmission line I, line ll,

.switch l3, filter F--3, primary of transformer T--6, filter F3, and line 12 to common lines 8 and 3. From that stage, the re-inversion of the received inverted speech proceeds in the manner already described.

Fig. 2 of the drawings shows the frequency translations of the original voice as it passes through the transmitting modulator, transmission line and receiving demodulator to the listener, The input voice band selected by the modulated with the carrier at 2800 cycles, the.

resultant modulated inverted voice is represented at b. When the carrier has shifted from 2800 to 3100 cycles, the inverted ,voice band shifts with it, as represented at c. The original band 400- 2400 cycles, as it varies between 400 and 2700 cycles, due to the 300 cycle shift of the carrier,

is represented at d. After this shifting band (400-2700 cycles) is demodulated with the shifting carrier of the receiver in a distant unit, the original speech band (400-2400 cycles) is reproduced, as represented at e. The carrier variation between 2800 and 3100 cycles, representing the selection width of the filter F2, is also represented at d. The 7th and 9th harmonics of the low frequency (350-387 cycles) current are rejected by the filter F2, since'it passes only the 8th harmonic, 2800-3100 cycles.

So far, in order to simplify the disclosure, only one low frequency shifting band (350-387 cycles) has been considered. In actual 'use, however, three separate shifting bands are used; namely,

(1) 233 to 258 cycles (2). 280 to 310 cycles (3) 350 to 387 cycles When (1) is used, the carrier band-selector filter F2 selects the 12th harmonic, which is 2800- 3100 cycles. When (2) is used, the filter selects the th harmonic, which is 2800-3100 cycles; and, when (3) is used the filter selects the 8th harmonic. I

These low frequency changes are automatically effected by the rotating contactor RC driven by the motor M. The contactor drive shaft is preferably geared to revolve once for every 40 revolutions of the condenser drive shaft. Therefore, after every forty shifts of the transmitted carrier (at irregularly varying speeds) the frequency'of the shifting'carrier band is changed to another shifting band. The result is that three separate and successively selected low frequency bands are transmitted to the line in addition to the inverted shifting voice band. These changing, shifting combinations occurring at random will cause complete mystification of eavesdroppers and create privacy transmission impossible to break. These are far more difficult combinations than the periodic scramblings used on trans-oceanic commercial service, for the present system produces an ever-changing scramble which cannot be separated by filters into a single band for-re-invcrsion.

The communication system is continuously i transmitting low frequencies in bands which vary between 233 and 387 cycles. No other frequencies appear on the line unless speech is being transmitted. No carrier current of 2800-3100 cycles appears on the line at, any time, since the carrier is balanced out of the line by the modulator, Hence no opportunity is given an eavesdropper for detecting the modulating frequency.

The elements of each privacy unit are so arranged that the transmitting (modulator) circuit does not feed into the receiving (demodulator) circuit. This construction greatly reduces the size and weight of the apparatus as compared with prior privacy systems in which this result is attained by the use of a hybrid or balanced bridge unit.

The magnitude of the useful harmonics of the low frequency shifting currents are about db,

below that of the fundamental, or are only about one-tenth of the original intensity. Hence a small amount of positive regeneration is given to the harmonic amplifier stage V-3 by coupling the 2000 ohm cathode resistor to its grid circuit. This additional gain provid'es for a wide range of transmission losses, which can be controlled precisely by the potentiometer P- 3 in the grid circuit of the amplifier V-3. The control rheostats R,I and R-2 are set with this grid potentiometer on mid-scale, so that if a transmission line of more than normal loss (as much as 20 db.) is encountered, a move of this grid potentiometer knob will raise the level of the 2800 to 3100 cycle current to normal. However, such condition as this will seldom be encountered on any communication circuit.

The utmost degree of privacy is secured by the action of the phase-shift network N (Figs. 4 and 5) which may be inserted in the generator section of each unit between 'the secondary of the transformer T-3 and the harmonic generator G. This network causes a time lag between the shifting low frequency current fed to the harmonic generator and that fed directly to the line through the low pass filter F3. 'The'result is that on the line the shifting low frequency current is out of phase with the inverted continuously shifting voice band.

The phase-shift network of the "active unit retards the generated harmonic with respect to the low frequency current passed directly to the line through the filter F3; hence, on the line the low frequency current and the inverted voice current are out of phase. At the receiving unit a similar network in the low frequencycircuit retards the received low frequency current to restore the original phase relationship with the inverted voice band to effect normal demodulation.-

There can be no possibility of an eavesdropper detecting the phase difference existing on the line. Since the actual modulating carrier (2800-3100 cycles) does not appear on the line. there will be no wave for comparison with any harmonic generated from the low frequency current. Therefore, no oscilloscope tests can detect or disclose these phase differences.

The network N is constructed to produce vary ing degrees of phase-shift. As shown in Figs. 4

the pivoted switch 2| across the contact terminals 22.

To facilitate adjustment of the switch 21, the network is provided with a scale 21 having indicia arranged as a scale for indicating the positions of the switch required to impose various degrees of phase shift on the generated low frequency currents. When the switch makes contact with the terminal No. l, the entire network is by-passed and the low frequency and inverted voice bands are transmitted through the line in phase. A move of the switch to terminal No. 2 imposes a lag of, for example, A; milLi-second, which means a frequency shift of cycles in the present case; that is, one-third of the total frequency shift.

Assuming that all the sections of the network have-the same phase-shift characteristics, it is obvious that a move of the switch to terminal No. 4 will cause the low frequency and inverted voice .'waves on the line to be 300 cycles out of -phase,

stant the other begins to decrease. The millisecond condition, imposed when the switch 2| contacts with terminal No. 5 will be similar to the milli-second condition, attained when the switch makes contact with No. 3 terminal, except that the waves move in opposite directions.

It will be obvious that all privacy units in the system must be set for the same phase shift in order to effect intelligible communication. It will also be apparent that any desired number of phase-shift sections may be employed in the networks.

It is evident that the general principles herein disclosed may be embodied in'many otherorganizations widely different from those illustrated without departing from the spirit of the invention.

What I claim is:

l. The method of secret telephony between stations of a communications system, which comprises: generating a low frequency current at one station and transmitting it through the line to the other station, generating a. harmonic of the low frequency current to form the same carrier at each station, modulating the carrier at either station in accordance with speech, selecting the lower side band from the modulated carrier, balancing the carrier out of. the selected side band, and transmitting the inverted speech current resulting from modulation of the carrier in accordance with speech to the line for reception by the other station and for demodulation with the carrier in that other station to reproduce the original speech.

2. The method of secret telephony between stations of a communications system, which comprises: generating a low frequency current at one station and transmitting it through the line to the otherstation, shifting the phase of the low frequency current, generating a harmonic of the phase-shifted low frequency current to form the same carrier at each station, modulating the carrier at either station inaccordance with speech, selecting the lower side band from the modulated carrier, balancing the carrier out of the selected side band, and transmitting the inverted speech current resulting from modulation of the carrier in accordance with speech to the line for reception by the other station and for demodulation with the carrier in that other station to reproduce the original speech.

3. The method of secret telephony between stations of a communications system, which comprises: generating a band of shifting low frequency current at one station and transmitting it through the line to the other station, generating a shifting band of harmonics of the shifting low frequency current to form the same shifting carrier band at either station, modulating the carrier band at either station in accordance with speech, selecting the lower side band from the modulated shifting carrier band, balancing the carrier band out of the selected side band, and transmitting the shifting inverted speech current band resulting from modulation of the. shifting carrier band in accordance with speech to the line for reception by the other station and for demodulation with the shifting carrier band at that other station to reproduc the original speech.

4. The method of secret telephony between stations of-a communication system, which comprises: generating a band of shifting low frequency current at one station and transmitting it through the line to the otherstation, shifting the phase of the low frequency current, generating a shifting band of harmonics of the phase-shifted low frequency current to form the same shiftin carrier band at each station, modulating the carrier band at either station in accordance with speech, selecting the lower side band from the modulated shifting carrier band, balancing the carrier band out of the selected side band, and transmitting the shifting inverted speech current band resulting from modulation of the shifting carrier band in accordance with speech to the line for reception by the other station and for demodulation with the shifting carrier band at that other station to reproduce the original speech.

5. In a secret signaling system; a privacy unit adapted to be detachably connected to a communications line and comprising a transmitting section including a modulator and a receiving section including a demodulator, said sections being arranged to prevent one section from transmitting into the other, and means for generating a carrier current for said modulator and demodulator.

6. The system set forth in claim 5 in which said means comprises a low frequency generator and a harmonic generator coupled to the low frequency generator and said modulator and demodulator to supply carrier current for each of said sections.

'7. In a secret signaling system, similar privacy units connected to a communications line, each unit comprising a transmitting section including a modulator and a receiving section including a demodulator arranged to prevent one section from transmitting into another, means for generating a low frequency current in one of said units and for transmitting it through the line to the other unit, and means in each unit for generating a carrier current for the modulator and demodulator of that unit from the said low frequency current.

3. A signaling system comprising a source of speech current, means for generating a shifting band of low frequency current and for inverting the frequencies of the various components of the speech current, means for transmitting energies corresponding to the energies of the shifting inverted and low frequency bands, means for receiving said energies at a distant point, and means controlled by the energy of the received low frequency band for re-inverting the frequencies of the inverted speech band to reproduce the original speech current.

9. A signaling system comprising a source of lowfrequency current and carrier waves, means for modulating said waves in accordance with speech, means for selecting the lower sideband of the modulated waves, means for balancing the carrier frequency out-of said side band, means for transmitting the energies of said side bandand low frequency current to a distant point, and means controlled by the received low frequency current for combining with the received side band energy to reproduce speech.

10. A signaling system comprising a source of for receiving said transmitted bands at said point, and means controlled by thereceived low frequency band for re-inverting the received speech band to reproduce speech.

11. In a secret signaling system, privacy units connected to a communications line, means for generating a low frequency current in one unit and transmitting it to the line, means in each unit for generating a harmonic of the low frequency current to form similar carrier current, means in each unit for modulating the carrier in accordance with speech, means for selecting the lower side band from the modulated carrier, means for balancing the carrier out of the side band, and means in each unit for transmitting the resultant inverted speech to the line for reception by and demodulation in the other units to reproduce the original speech.

12. In a secret signaling system, privacy units connected to a communications line, means for generating a low frequency current in one unit and transmitting it to the line, means in each unit for shifting the phase of the low frequency current, means in each unit for generating a harmonic of the phase-shifted low frequency current to form a carrier current, means in each unit to modulate the carrier in accordance with speech, means for selecting the lower side band from the modulated carrier, means for balancing the carrier out of th side band, and means in each unit for transmitting the resultant inverted speech current to the line for reception by and demodulation in the other unit to reproduce the original speech. a

13. In a secret signaling system, privacy units connected to a communications line, means for generating a continuously shifting band of low frequency current in one of said units and for transmitting that band to the line, means in each unit for shifting the phase of the low frequency currents, means in each unit for generating a shifting band of harmonics of the phase-shifted band of shifting low frequency current, to form a shifting band of carrier current. means in each unit for modulating the shifting band of carrier current in accordance with speech, means for selecting the lower side band from the modulated shifting carrier band, means for balancing the carrier out of the side band, and means in each unit for transmitting the resultant shifting inverted speech band to the line for reception by and demodulation in the other unit to reproduce the original speech.

14. In a secret signaling system, privacy units connected to a, communications line, means for generating a continuously shifting band of low for reception by and demodulation in the other unit to reproduce speech.

15. In a secret signaling system, privacy units connected to a communications line, means for generating a low frequency current in one of said units and means for transmitting it to the said line, and means in each unit for generating a carrier'current from the generated or received low frequency current.

16. In a secret signaling system, privacy units connected to a communications line, means for generating a low frequency current in one of said units and means for transmitting it to the said line, means in each unit for shifting thephase of the generated or received low frequency current, and means in each unit for generating a carrier current from the phase shifted low frequency current.

1'7. In a secret signaling system, privacy unitsv connected to'a communication line, each unit having a circuit including an electronic tube and a manually controllable switch in the tube circuit to'adapt the tube for use alternatively as a current generator in any selected one of said units and as an amplifier in each of the remaining units for the current generated in the selected unit. EDWIN YATES WEBB, JR. 

